Water Heater and a Method of Operating Same

ABSTRACT

A water heater (10) including a tank (12), a burner (26) and a heated gas outlet duct (40). The tank (12) has a substantially central flue (14) having an outlet (24) substantially adjacent the top (20) of the tank (12). The tank (12) is adapted for mounting with the flue (14) substantially vertically. The burner (26) is located beneath the tank (12). The burner (26) has an air inlet (32) and a pilot flame and is adapted to a burn a fuel to discharge heated gases up through the flue (14). The heated gas outlet duct (40) is of substantially unrestricted cross-section and has an upper, first end (42) in gas communication with the flue outlet (24) and a second, lower end (46) in gas communication with atmosphere. When only the pilot flame is burning, the heated gas outlet duct (40) is adapted to restrict the air drawn into the flue (14) via the air inlet (32) to the minimum amount required to maintain the pilot flame alight.

FIELD OF THE INVENTION

The present invention relates to a water heater and a method of operating same.

The invention has been primarily developed for use in a natural draft water heater that has a gas burner at the base of the water tank and a flue which passes through the water tank for heating water in the tank.

BACKGROUND OF THE INVENTION

Natural draft water heaters are well known and are used extensively in domestic water heating and other applications. The energy efficiency of such water heaters is considered to be an important consideration by many purchasers and owners of such water heaters.

The Australian Gas Association (AGA) conducts performance testing for water heaters and generates a Star (Trade Mark) rating indicating the heater's overall energy efficiency (e.g. 5 Star). The higher the Star rating, the higher the overall energy efficiency. The Star rating is, generally speaking, a combination of: Efficiency; and Maintenance. Efficiency is a measure of the energy transfer from fuel to heated water (i.e. heat exchanging efficiency). Maintenance is a measure of retaining stored energy in the water (i.e. insulation effectiveness).

Natural draft water heaters operate in two modes: a ‘heating’ mode when the main burner is operating (i.e. alight); and an ‘idle’ mode when the main burner is not operating. In water heaters that have a constantly operating pilot flame, only the burner's pilot flame is alight in idle mode. In water heaters that have an electronically controlled gas valve that re-ignites the main burner electronically, there is no pilot flame alight in idle mode.

Further, a water heater with a pilot flame and an open flue (i.e. a flue vented directly to atmosphere without restriction) will, in idle mode, transfer approximately 60% of the pilot flame's energy to the water and lose approximately 40% through the flue. Attempts to minimise this energy loss include placing a damper (i.e. a flow control valve) downstream of the flue which is controlled to block the flue opening to atmosphere when in idle mode.

However, disadvantages of this approach are that the damper adds to the cost and complexity of the water heater and can also create a safety hazard if it jams in the closed position when the heater is in the heating mode.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a mechanically simple, relatively low cost water heater able to achieve a 5 Star AGA energy rating.

SUMMARY OF THE INVENTION

Accordingly, in a first aspect, the present invention provides a water heater including:

-   -   a tank with a substantially central flue having an outlet         substantially adjacent the top of the tank, the tank being         adapted for mounting with the flue substantially vertical;     -   a burner located beneath the tank, the burner having an air         inlet and a pilot flame and being adapted to a burn a fuel to         discharge heated gases up through the flue; and     -   a heated gas outlet duct of substantially unrestricted         cross-section having an upper, first end in gas communication         with the flue outlet and a second, lower end in gas         communication with atmosphere,     -   wherein, when only the pilot flame is burning, the heated gas         outlet duct is adapted to restrict the air drawn into the flue         via the air inlet to the minimum amount required to maintain the         pilot flame alight.

The length of the duct is preferably approximately 20% to 30%, most preferably 25%, of the length of the flue. The duct and the flue are preferably cylindrical and have approximately equal diameters.

In a second aspect, the present invention provides a water heater including:

-   -   a tank with a substantially central flue having an outlet         substantially adjacent the top of the tank, the tank being         adapted for mounting with the flue substantially vertical;     -   a burner located beneath the tank, the burner having an air         inlet and an electronically controlled ignitor and being adapted         to a burn a fuel to discharge heated gases up through the flue;         and     -   a heated gas outlet duct of substantially unrestricted         cross-section having an upper, first end in gas communication         with the flue outlet and a second, lower end in gas         communication with atmosphere,     -   wherein, the duct has approximately the same cross sectional         area of the flue and is approximately 25% of the length of the         flue.

The duct and the flue are preferably cylindrical.

In a third aspect, the present invention provides method of maintaining a pilot flame alight in a water heater having a tank with: a bottom; a top; and a substantially central flue, the method including the steps of:

-   -   directing the heated gases from the pilot flame from the tank         bottom to the tank top via the flue; and     -   directing the heated gases from the tank top to atmosphere via a         generally downwardly directed duct of substantially unrestricted         cross-section, the duct being adapted to restrict the gas flow         drawn into the flue to only the minimum amount required to         maintain the pilot flame alight.

In a fourth aspect, the present invention provides a method of operating a water heater in idle mode, the water heater having a tank with: a bottom; a top; a burner with a pilot flame; and a substantially central flue, the method including the steps of:

-   -   directing the heated gases from the pilot flame from the tank         bottom to the tank top via the flue; and     -   directing the heated gases from the tank top to atmosphere via a         generally downwardly directed duct of substantially unrestricted         cross-section, the duct being adapted to restrict the gas flow         drawn into the flue to only the minimum amount required to         maintain the pilot flame alight.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, by way of examples only, with reference to the accompanying drawing wherein:

FIG. 1 is a cross-sectional schematic side view of an embodiment of a water heater according to a first embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a natural draft water heater 10 according to a first embodiment of the invention. The water heater 10 includes a cylindrical steel water tank 12 with a central flue 14. The flue 14 has a length L1 and a spiral baffle 16 therein. The tank has an outwardly concave (domed) bottom 18 and an outwardly concave (domed) top 20. The flue 14 has an inlet 22 adjacent the tank bottom 18 and an outlet 24 slightly above the tank top 20. In use, the tank 12 is almost completely filled with water.

It will be well understood by those skilled in the art that cold water is introduced from mains water supply via a cold water inlet (not shown) near the tank bottom 16 of the tank 12 and hot water is drawn off for use via a hot water outlet (not shown) near the top 20 of the tank 12.

A burner 26 is located beneath the tank bottom 18. As is well understood by persons skilled in the art, the burner 28 has a pilot flame (not shown) and is adapted to burn fuel, preferably natural gas, and discharge heated gases up through the flue 14 for heat transfer to the water in the tank 12. The exterior of the tank 12 is surrounded by insulation 28. The tank 12 and the insulation 28 are contained within an outer casing 30, which includes an air inlet 32. The air inlet 32 is in gas communication with the burner 26 via passage 34. The burner 28 also has an associated gas control system 36.

The water heater 10 also includes a heated gas outlet duct 40 having a first upper end 42 connected to the outlet 22 of the flue by an insulated U-shaped connector 44 and a lower second end 46 open to atmosphere. The duct 40 has a length L2 and has a substantially open or unrestricted cylindrical cross-section (i.e. free of valves or dampers and the like). The duct 40 is adapted to provide a resistance to the gas flow therethrough such that when the water heater is in idle mode (i.e. only the pilot flame is burning) the gas passing through the duct 40 is subjected to a balancing resistance, which limits the air drawn into the flue 14 to only the minimum that is required to achieve combustion and maintain the pilot flame alight. Generally speaking, as a result, the energy that would ordinarily be lost, in idle mode, through an unrestricted outlet is minimised and the Star rating of the water heater is improved, particularly the Maintenance-based component of the Star rating.

The resistance of the duct 40 is primarily adjusted by varying its diameter and length. The diameter of the duct 40 is approximately equal to that of the flue 16 to avoid choking the gas flow through the flue/duct. In the embodiment shown, the duct length L2 is approximately 25% of the flue length L2. Further, it will be appreciated by those persons skilled in the art that other factors can influence the resistance provided by the duct 40 including material and internal surface finish.

Generally speaking, the relative length of the duct 40 is determined by the following iterative process. Firstly, the gas supply for the pilot flame is established as the minimum amount possible to meet environmental (e.g. wind) and legislative requirements. Secondly, the water heat 10 is tested, without the duct 40 attached, and the restriction of the baffle 16 adjusted to meet emission requirements and maximise the Efficiency-based Star rating component. The duct 40 is then attached. As discussed above, this improves the Maintenance-based Star rating component. However, it also generally lead to a reduction in the Efficiency-based Star rating component. The baffle 16 is then adjusted, by making it less restrictive, to increase the Efficiency-based component until an acceptable level of emissions is reached. The length of the duct 40 is then adjusted to seek improvements in the Maintenance-based component. This process continues in an iterative manner until the overall Star rating is maximised.

The water heater 10 provides improved efficiency as the duct 40 increases the transfer of heat from the pilot flame to the water when the heater 10 is in idle mode by reducing heat loss from the flue 14 to atmosphere. This improves the Maintenance-based Star rating component. More particularly, it is estimated that water heaters similar to that described with reference to the water heater 10 will, in idle mode, transfer approximately 80% of the pilot flame energy to the water and only lose approximately 20% through the flue. Further, such water heaters do not exhibit a loss in their Efficiency-based Star rating component sufficient to compromise their Star rating overall. Moreover, this is achieved in a relatively mechanically simple arrangement that is not susceptible to blocking or other mechanical failure.

A water heater (not shown) according to a second embodiment of the invention is similar to the heater 10 except it has an electronically controlled gas valve that re-ignites the main burner electronically. This heater has no pilot flame alight in idle mode. The duct 40 also improves the Star rating of this heater as it provide a resistance to, and thus slows, the flow of the heated air (ie. air heated by transfer from the surrounding water) leaving the flue.

Although the invention has been described with reference to the preferred embodiments, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. 

1. A water heater including: a tank with a substantially central flue having an outlet substantially adjacent the top of the tank, the tank being adapted for mounting with the flue substantially vertical; a burner located beneath the tank, the burner having an air inlet and a pilot flame and being adapted to a burn a fuel to discharge heated gases up through the flue; and a heated gas outlet duct of substantially unrestricted cross-section having an upper, first end in gas communication with the flue outlet and a second, lower end in gas communication with atmosphere, wherein, when only the pilot flame is burning, the heated gas outlet duct is adapted to restrict the air drawn into the flue via the air inlet to the minimum amount required to maintain the pilot flame alight.
 2. The water heater as claimed in claim 2, wherein the length of the duct is approximately 20% to 30% of the length of the flue.
 3. The water heater as claimed in claim 1, wherein the length of the duct is approximately 25% of the length of the flue.
 4. The water heater as claimed in claim 1, wherein the duct and the flue are cylindrical.
 5. The water heater as claimed in claim 4, wherein the duct and flue have approximately equal diameters.
 6. A water heater including: a tank with a substantially central flue having an outlet substantially adjacent the top of the tank, the tank being adapted for mounting with the flue substantially vertical; a burner located beneath the tank, the burner having an air inlet and an electronically controlled ignitor and being adapted to a burn a fuel to discharge heated gases up through the flue; and a heated gas outlet duct of substantially unrestricted cross-section having an upper, first end in gas communication with the flue outlet and a second, lower end in gas communication with atmosphere, wherein, the duct has approximately the same cross sectional area of the flue and is approximately 25% of the length of the flue.
 7. The water heater claimed in claim 6, wherein the duct and the flue are cylindrical.
 8. A method of maintaining a pilot flame alight in a water heater having a tank with: a bottom; a top; and a substantially central flue, the method including the steps of: directing the heated gases from the pilot flame from the tank bottom to the tank top via the flue; and directing the heated gases from the tank top to atmosphere via a generally downwardly directed duct of substantially unrestricted cross-section, the duct being adapted to restrict the gas flow drawn into the flue to only the minimum amount required to maintain the pilot flame alight.
 9. A method of operating a water heater in idle mode, the water heater having a tank with: a bottom; a top; a burner with a pilot flame; and a substantially central flue, the method including the steps of: directing the heated gases from the pilot flame from the tank bottom to the tank top via the flue; and directing the heated gases from the tank top to atmosphere via a generally downwardly directed duct of substantially unrestricted cross-section, the duct being adapted to restrict the gas flow drawn into the flue to only the minimum amount required to maintain the pilot flame alight. 